This invention relates to a polymeric composition and a method for preparing this composition. More particularly, this invention relates to an epoxy adhesive composition having excellent shear adhesion to metallic substrates and a method for preparing this composition.
Epoxy resins are, of course, well known in the prior art. These resins have a spectrum of properties that are well adapted for use in molding compositions, coatings, adhesives, fiber reinforced laminates, composites, engineering plastics and various speciality applications such as potting resins and mortars. Included in this spectrum of properties are good corrosion and solvent resistance, good adhesion, good electrical properties, good dimensional stability, hardness, low shrinkage on cure and many other beneficial properties. Unfortunately, however, epoxy resins suffer a major disadvantage in that the epoxy resins are, generally, undesirably brittle.
Several attempts have been made to produce an epoxy resin composition retaining or at least substantially retaining the desirable epoxy resin properties while reducing the brittleness or improving the impact resistance thereof. Early attempts to accomplish this objective involved dissolving a reactive liquid polymer such as a carboxyl-terminated butadiene-acrylonitrile copolymer into an epoxy prepolymer as discussed in the background sections of U.S. Pat. No. 4,707,529 and Japanese Patent (Kokai) No. SHO 58[1983]-17160. As indicated in the background section of '529, incorporation of the reactive liquid polymer led to the formation of discrete elastomer particles or domains which, upon cure, toughened the epoxy resin. As is now known, however, the resulting composition will exhibit a lower softening temperature and a lower glass transition temperature than the unmodified epoxy resin. Also, while improved mechanical properties, particularly toughness, are generally obtained, it is difficult to control the extent and consistency of the improvement. This result is apparently due to the difficulty of controlling the morphology of this particular composition.
Another attempt to improve the toughness of compositions comprising polyepoxides with liquid polymers is also discussed in '529. In this second process, liquid acrylate rubbers are incorporated into epoxy resin compositions for the purpose of improving the composition's impact resistance without adversely affecting desirable properties. Such compositions are the subject of European Patent Application No. 78,527. The acrylate rubbers are, however, generally, soluble in the polyepoxide continuous phase at temperatures above about 51.degree. C. and in some cases even at room temperature. As a result, undesirable softening of the cured epoxy resin composition can readily occur. In addition, it is again difficult to control the size of the particles of the dispersed modifying phase, and hence, the morphology of the cured composition. This is because the polymeric phases do not separate until the epoxy cures, and the dissolving and reprecipitation of these phases are sensitive to the manner in which the curing takes place.
Another attempt to improve polyepoxide compositions involved the incorporation of a normally solid, selectively hydrogenated, modified block copolymer in the composition. The block copolymer contained at least one aromatic vinyl compound polymer block and at least one hydrogenated conjugated diolefin polymer block. The polymer was modified by grafting dicarboxylic acid or carboxylic acid derivative (e.g. anhydride) functionality into the conjugated diolefin polymer blocks. Such compositions are taught in Japanese Patent (Kokai) No. 58[1983]-17160 and Application Publication No. 57[1982]-149369.
Japanese Application Publication '369 teaches an adhesive composition comprising 100 parts by weight of a carboxylic acid modified block copolymer of a conjugated diolefin and a vinyl arene; and 5 to 100 parts by weight of an epoxy resin. The method and apparatus for mixing the component is said to be not critical, and examples are described where the epoxy resin and modified block copolymer are mixed by kneading for 15 minutes at 120.degree. C.
The compositions of '369 are high in rubber content and would not exhibit properties generally associated with polyepoxide compositions to any great extent. The compositions of '369 could be characterized as epoxy crosslinked modified block copolymers, which differ from the epoxy-modified block copolymer compositions which have continuous epoxy phases. Also, epoxy curing agents are not utilized because the epoxy functions in these compositions to crosslink the modified block copolymers, and the epoxy groups are not then available for subsequent curing.
A heat curable epoxy composition is taught in Japanese Application '160. The heat curable epoxy composition comprises a resin selected from a group which includes epoxies, and a carboxylic acid modified block copolymer of a conjugated diolefin and a vinyl arene. The examples in '160 are prepared by mixing 100 parts by weight of an epoxy with 10 parts by weight of a modified block copolymer. The modified block copolymer was freeze pulverized to a size of 80 mesh or less. The mixture was then mixed at 150.degree. C. for 30 minutes. Applicant has found that adhesive compositions of 10% modified copolymers in 90% epoxies which had been prepared by mixing freeze pulverized modified block copolymers into the epoxy had lap shear strengths (as measured by ASTM 1002 using an anodized aluminum substrate) which were not significantly greater than lap shear strengths of epoxy compositions without modified copolymers. For adhesive applications, the process of preparing the epoxy-modified block copolymer compositions taught in '160 is therefore not advantageous. Incorporation of polymers of this type into an epoxy resin composition did, according to the disclosure of '160, further improve the impact resistance as represented by the notched Izod impact strength.
Still another method of improving epoxy compositions is taught in '529. In this particular method, an epoxy resin composition comprising a dispersion of siloxane polymers and a polyepoxide is provided. At least a portion of the siloxane polymer is actually grafted to the epoxy resin. These compositions exhibit improved impact resistance and improved thermal and mechanical properties. The epoxy resin must, however, be modified to facilitate the grafting reaction. This, then, could adversely affect the desired properties of the polyepoxide. Moreover, a cocatalyst is required to effect at least certain of the necessary reactions in this process.
Since epoxy resins are well adapted for a broad range of uses, particularly adhesive compositions, the lap shear strength could be increased without significant loss of any of the desirable properties of the epoxy resin including the thermal properties thereof, particularly on a consistent, controllable basis, and since the prior art methods designed to accomplish this objective have not been completely successful the need for an improved method or composition to accomplish this objective is believed to be readily apparent. It is particularly desirable to provide an epoxy-elastomeric block copolymer composition wherein the elastomer domains form stable dispersions prior to curing of the epoxy so the size and shapes of the domains are not significantly effected by the curing conditions.